1. Field of the Invention
The present invention relates to an aqueous photocurable resin composition containing a novel aqueous polymerizable compound, an ink, an ink-jet recording method using the ink, an ink cartridge, a recording unit and an ink-jet recording apparatus. In the field of resinous painting technology, conversion to aqueous paints or inks which do not use organic solvents in a large quantity and are much more ecology-minded is one of the important problems.
The present invention is directed to a useful resin composition which meets such a demand. The resin composition of the present invention can be used as a material for a UV curable aqueous ink for use by an ink-jet apparatus. The present invention also provides an anionic polymerization initiator which has a high water solubility.
2. Related Background Art
As one of the techniques related to screen inks, gravure inks, or aqueous paints, there is a process for forming an image in which a resin in an ink or paint is cured by ultraviolet light irradiation, and the ink or paint is often an aqueous ink or paint. Such an aqueous paint or ink, however, contains a non-aqueous UV curable resin composition emulsified in an aqueous solvent. Thus, the UV curable resin composition used in aqueous paint or ink in the prior art is not itself an aqueous system and it is not going too much to say that aqueous UV curable resins and catalysts have not been pursued.
Use of a UV curable ink is also known in the ink-jet printing process. In this field, also, the resin composition used is a non-aqueous system, for example, an oil-ink in which a pigment is dispersed in an organic solvent such as toluene, methylethyl ketone, etc. For marking, an ink comprising resin monomer and oligomer and a pigment dispersion not using solvent is used. Japanese Patent Publication 5-64667 discloses a typical example of such an ink. However, since the ink viscosity of these inks is too high to print precise letters, their use is limited to dot printing such as marking which does not require high precision.
Inks employed in the aforementioned systems do not possess ink properties required for full color printing with high-resolution. For example, the oil-ink should be used with care from the environmental point of view, and the solvent-free UV-curable ink is usable only for a low-resolution printer of large dot size. U.S. Pat. No. 4,978,969 discloses an ink for ink-jet recording wherein a certain kind of a polymerizable monomer is used in a large amount as a solvent to dissolve the UV-curable adhesive. Although application to a thermal ink jet system is also intended in the above invention, actually, materials are limited and the monomer is used in a large amount, so that it is not all-purpose and not applicable for high quality image formation.
The reason why the use of UV curable ink remains in the conditions as described above in the art may be considered as follows:
1) Images formed with an ink or paint comprised of a resin dissolved in water do not have sufficient quality even after cure because of poor water-resistance and low gross; and
2) Aqueous UV curable resins or aqueous photopolymerization initiators having a catalytic function to assist curing of such a resin (hereinafter, referred to as xe2x80x9ccatalystxe2x80x9d) have hardly been developed.
In addition, one may point out another reason for slow development in material technology of aqueous UV-curable inks:
3) When water is used as a paint solvent, relatively a large amount of energy is required for drying in comparison with an organic solvent, which is a drawback in practical use.
On the other hand, the above mentioned resin composition in an emulsion state generally has an advantage that viscosity and fluidity can be controlled to vary in a wide range, and handling is easy.
However, the UV curing technique is still regarded as a promising curing technique in view of energy saving, low environmental contamination, and low burden to the environment. Further, the UV-curing process is considered useful for not only image printing but also for pretreatment of a substrate to provide printing suitability, or for post-treatment of the printed substrate to protect it or to apply a certain material for further processing.
At present, aqueous materials such as the above mentioned aqueous UV curable resins and catalysts are hardly available for an ink-jet recording ink. Here, to be used in an ink-jet system, materials having a good fluidity and a low viscosity are required to cope with the high-density nozzles. For example, the following properties are required for resin materials such as catalysts and polymerizable materials: the polymerizable compound can be added to an ink at a high content; substantial drying process can be shortened, the cured layer formed by applying the ink followed by curing has excellent properties; and the resin material show a good compatibility with coloring materials.
The UV curable water-soluble materials practically in use include those having both of an acidic group and (meth)acryloyl or vinyl group in one molecule, and as a water-soluble polymerizable compound, an ester of anhydrous succinic acid and 2-hydroxyethyl(meth)acrylate, an ester of anhydrous ortho-phthalic acid and an ester of 2-hydroxyethyl(meth)acrylate, and vinylnaphthalene sulfonic acid are actually in use. However, these compounds have only one functional group per molecule, thus, the polymerization speed is slow and crosslinking degree is extremely small, so that they cannot be used as a main material for the UV-crable resin composition.
In the meantime, polymerizing compounds being water-soluble, having at least two polymerizable functional groups per molecule and being produced in an industrial scale include those having a polyethyleneoxide chain which provides hydrophilicity to the molecule. For example, it can be mentioned a (meth)acrylic acid ester of polyvalent alcohol such as diethyleneglycol di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, etc. According to the present inventors"" investigation, however, the aforementioned compounds lack water-solubility when the ethyleneoxide chain is short, and when the ethyleneoxide chain is long, the water solubility is sufficient but the solid properties such as hardness or adhesiveness of the polymerized or cured resin is not sufficient enough to satisfy the requirements for a paint or ink
Japanese Patent Application Laid-Open No. 8-165441 discloses water-soluble polymerizable polyfunctional compounds. The compounds disclosed in this publication have got water-solubility due to the increased number of hydroxy groups in a molecule.
Japanese Patent Application Laid-Open No. 2000-117960 discloses practical use of (meth)acrylic acid ester of a hydrophilic polyepoxide derived from glycerin. However, according to the study of the present inventors, although these compounds are excellent in UV polymerization ability and in physical properties of cured product, they have a problem in that the viscosity of an aqueous solution is somewhat higher than that required for an ink-jet ink.
Although various compounds having a basic atomic group are industrially produced such as monofunctional tertiary amines and cationic acrylic monomers, at present, few compounds are present in an industrial level which are basic and polyfunctional. Concerning polyfunctional, water-soluble, polymerizing compounds, U.S. Pat. No. 5,612,388 and Japanese Patent Application Laid-Open Nos. 8-165441 and 2000-117960 propose synthesis of polyfunctional cationic acrylic monomers and application thereof. These cationic polyfunctional compounds however, have an opposite polarity to that of an aqueous pigment dispersion being suitable as a coloring material for inks and paints where a pigment is homogeneously dispersed in an aqueous medium by the action of dissociable cationic groups. Thus, they do not have a compatibility with the pigment dispersion and cannot be applied to a coloring composition such as paint or ink other than some specific use.
Properties desired for a UV curable resin composition material useful for ink-jet recording ink are: when it is contained in an ink, the ink has a good ejecting property from the ink-jet recording head, rapidly polymerizes under UV irradiation; and is highly crosslinked and has superior hardness and adhesiveness when cured. Thus, such a UV curable resin composition material is a water soluble compound having low viscosity and multiple functional groups.
On the other hand, compounds known in the prior art to have such properties do not ionize in an aqueous solution but their water-solubility is provided by introducing numerous hydroxyl groups. Japanese Patent Application Laid-Open No. 8-165441 discloses this kind of compounds. But, these compounds whose water-solubility is not due to ionization have problems that the molecular size is liable to be large, and even in the cross-linked solid state, hardness and adhesiveness are not still satisfactory. For a rapid polymerization rate, plural polymerizable functional groups are needed. However, in order for a compound to have multiple polymerizable functional groups, water-solubility and stability at the same time, the compound has to satisfy requirements not compatible with each other. This is the reason why such an ideal compound has not been found yet.
Therefore, it is an object of the present invention to provide a UV curable resin composition of high performance.
It is also an object of the present invention to provide a photopolymerizable aqueous ink which is able to form high-quality images.
Another object of the present invention is to provide an ink-jet recording apparatus for forming high-quality images, a recording unit and ink cartridge utilized therefor.
Yet another object of the present invention is to provide a photopolymerizable initiator of high water-solubility which can be adequately used in a photocurable aqueous resin composition.
According to an embodiment of the present invention, it is provided an aqueous photocurable resin composition comprising at least water, a polymerizable material which polymerizes with radial initiation and a water-soluble photopolymerization initiator which generates radicals with light, wherein the polymerizable material has at least two polymerizable functional groups and one or two anionic functional groups.
According to another embodiment of the present invention, there is provided an aqueous ink which comprises the above mentioned an aqueous photocurable resin composition as a main component.
According to still another embodiment of the present invention, there is provided an ink-jet ink which comprises the aforementioned aqueous photocurable resin compostion.
According to still another embodiment of the present invention, there is provided a recording unit comprising an ink container containing the above ink-jet ink, and a head portion from which the ink is ejected.
According to still another embodiment of the present invention, there is provided an ink-jet recording apparatus comprising an ink container containing the above ink-jet ink, and a head portion from which the ink is ejected.
According to still another embodiment of the present invention, there is provided a photopolymerization initiator comprising a compound represented by Formulae TX1, TX2 or TX3: 
wherein, R2 is a group selected from the group consisting of xe2x80x94(CH2)xxe2x80x94 (x=0 or 1), xe2x80x94Oxe2x80x94(CH2)yxe2x80x94 (y=1 or 2), and substituted or unsubstituted phenylene, M is a hydrogen atom or an alkaline metal, R3 and R4 independently represent a hydrogen atom or a group selected from the group consisting of substituted or unsubstituted alkyls, and m is an integer from 1 to 10.